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A Comparison of Martian and Terrestrial Impactite Dykes

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A Comparison of Martian and Terrestrial Impactite Dykes Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5063.pdf A COMPARISON OF MARTIAN AND TERRESTRIAL IMPACTITE DYKES. E. A. Pilles1, L. L. Torna- bene1, and G. R. Osinski1. 1Centre for Planetary Science and Exploration / Dept. of Earth Sciences, University of Western Ontario, London ON. Introduction: Impactite dykes consisting of im- that typically corresponds to the surrounding target pact melt rocks and/or breccias occur in nearly every rocks [1,4], suggesting they formed from in-situ fric- terrestrial impact crater studied. They can be hosted in tional melting and/or cataclasis (parautochthonous the crater floor, walls, or central uplift, and often occur breccias), with some transportation involved. as complex networks of dyke systems that formed as a Lithic breccia dykes consist of a fine-grained clastic result of the impact event. Various mechanisms have matrix, and include both polymict and monomict brec- been proposed for the formation of these dykes, such as cias. They vary considerably in size from only 10s of the injection of impact melt into fracture systems (ei- cm’s thick (e.g., at the Manicuagan impact structure ther caused by the impact or pre-existing weaknesses) [1]) to 10s of meters thick (e.g., the Slate Islands im- [1], in-situ melting [2] or physical breakdown (catacla- pact structure [5]. Dykes of this nature have been pro- sis) during the impact event, or simply the uplifting posed to have formed during the modification stage of pre-existing dykes. However, for craters on Earth, in crater formation [4]. many cases there is limited surface exposure, due to the extent of degredation and/or exposure of the impact Impact melt rock dykes are continuous dykes that structure, and interpretations of the dykes’ formation range from a few mm in width to several 10s of meters are, therefore, often limited to sparse outcrop expo- thick. These dykes commonly contain lithic, mineral, sures or drill core. For these reasons, this study exam- and glass fragments in a matrix of impact melt material ines remote sensing data from exposed bedrock in the nd are believed to have formed during late-stage frac- central uplifts of Martian impact craters to compare the turing during the crater modification process [4]. morphology of Martian impactite dykes to potential On a different scale altogether are impact melt terrestrial analogues. dykes that range from ~10 to 100 meters wide and up Methodology: A total of 1,338 central uplifts were to 50 km in length that occur in large impact structures, surveyed using images from the HiRISE (High- such as the so-called Offset Dykes at the 150 – 200 km resolution Imaging Science Experiment) camera on diameter Sudbury impact structure [6] or the grano- board the Mars Reconnaissance Orbiter (MRO). This is phyre dykes at the ~300 km diameter Vredefort impact a continuation of the global crater-exposing bedrock structure [2]. They occur as planar sheets and/or dis- (CEB) database developed by [3]. Of these, 416 central continuous bodies within the basement rocks and occur uplifts had exposed bedrock. Of those, dykes were ob- concentrically around – or extending radially outwards served in 96 central uplifts. This does not imply that from – the impact melt sheet. In this study, dykes of there are no dykes in other craters, merely that there is this scale are referred to as offset-style dykes. a lack of exposure in other craters. Dykes were catego- Discussion: The dykes were categorized based on rized based on their morphology, and comparisons their colour and morphology in HiRISE images: were made to terrestrial examples. Thin dark-toned fractures are dark-toned dykes Terrestrial impactite dykes: In spite of their lim- typically <5 m thick that form complex anastomosing ited surface exposure, terrestrial impactite dykes have networks, and occasionally contains m-scale clasts. been classified in various ways based on their mor- This type of dyke was ubiquitous across all uplifts that phology (e.g., branching vs anastomosing), the texture contained fractured bedrock (see the thin, dark frac- and composition of their matrix (e.g., melt matrix vs tures in Fig. 1A). A terrestrial analogue for this type of clastic matrix), and their clast content (e.g., clast-rich dyke is pseudotachylite, which similarly consist of a vs clast-free; or polymict vs monomict) [4]. Although dark-toned matrix, and are known to form complex observations of the matrix of Martian dykes are not networks in the footwall and central uplifts of impact plausible with remote sensing data, the morphology structures [1]. At the resolution of HiRISE, clasts <1 m and clast content can be observed with the HiRISE in size would be difficult to resolve. (High-resolution Imaging Science Experiment) camera on board the Mars Reconnaissance Orbiter (MRO). For Dark-toned breccia dykes are characterized by a the purposes of this study, we consider four dyke types: dark-toned matrix that contains breccia clasts of vary- ing size, some of which are exotic in origin, indicating Pseudotachylites contain cm-scale (and occasional- some degree of transport was involved (Fig. 1A). They ly m-scale) clasts in a melt matrix, with a composition are typically ~5 – 50 m thick, fairly straight, with sharp Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5063.pdf contacts with the surrounding rocks. In terrestrial im- ficult to draw direct comparisons between these dykes pact structures, pseudotachylites can form large irregu- and terrestrial analogues. lar bodies, some 10s of meters thick, often containing Offset-style dykes: We identified only 7 examples m-scale clasts, and can be continuous for up to ~100 m of Martian impactite dykes >50 m wide. It is possible (for example, at the Vredefort impact structure). This that dykes of this scale exist in other craters, but are not variety of pseudotachylite is potentially analogous to observable due to dust or poor exposure. The morphol- the dark-toned breccia dykes in Martian craters. ogy of these dykes can be compared directly to the Light-toned and blue-toned dykes are typically <10 morphology of the Offset Dykes at Sudbury. The dykes m thick, up to 1 km in length, and have sharp contacts are commonly offset by 10s or 100s of meters, and with the surrounding rocks (Fig. 1B). These dykes form branches that weave in and out giving the dykes commonly have a sinuous shape, and stand out in relief an anastomosing geometry (Fig. 1C). forming ridges. Some appear blue-toned in HiRISE Conclusions: Dykes in terrestrial impact structures IRB images, indicative of ferrous-bearing material. It is share many similarities in morphology to those ob- difficult to find a direct comparison between this group served in Martian impact structures. And while offset- of dykes and a single terrestrial impactite dyke. One style dykes are poorly exposed (only 7/1,338 craters potential analogue to the light-toned dykes is lithic surveyed contained them), there are Martian examples breccia dykes, which vary considerably in size from of these features. Further study of dykes in well- only 10s of cm’s thick (e.g., at the Manicuagan impact preserved Martian craters will aid our understanding of structure [1]) to 10s of meters thick (e.g., the Slate Is- similar features in terrestrial craters that are heavily lands impact structure [4]. Another potential analogue eroded, metamorphosed, and difficult to interpret. to the light-toned dykes is impact melt rock dykes. These dykes are straight, continuous dykes that range References: [1] Dressler and Reimold (2004) from a few mm in width to several 10s of meters thick. Earth-Sci Rev, 67, 1–54. [2] Reimold and Gibson These dykes commonly contain lithic, mineral, and (2011) Chem Erde-Geochm, 66, 1–35. [3] Tornabene et glass fragments in a matrix of impact melt material that al. (2015) Bridging the Gap III, 1861, p. 1043. [4] typically reflects the composition of the dyke’s imme- Lambert (1981) In Multi-ring basins: Formation and diate host rocks. When found in crystalline targets, evolution, pp.59–78. [5] Fairchild, Swanson-Hysell, these dykes typically have a black to dark-grey matrix, and Tikoo (2016) GSA, 44, 723–726. [6] Grant and that is not consistent with their light-toned appearance Bite (1984) In The geology and ore deposits of the in HiRISE images (e.g., the Mistastin Lake [7] and Sudbury structure, pp. 275–300. [7] Singleton et al. West Clearwater impact structures [8]). However, (2011) LPSC, 42, p. 2250. [8] Wilks (2016) M.Sc. when dykes of this scale occur in sedimentary targets, Thesis, UWO. [9] Wittmann et al. (2004) Meteoritics they tend to have a lighter-coloured matrix than the & Planet. Sci., 39, 931–954. [10] Reimold et al. (2005) crystalline-hosted dykes (e.g., at the Chicxulub [9], Meteoritics & Planet. Sci., 40, 591–607. [11] Sharpton, Manicuoagan [1], Siljan [10], and Terny impact struc- Krochuk, and Herrick (2013) Meteoritics & Planet. tures [11]). However, most of the light-toned dykes Sci., 48, 806–818. observed in this study do not contain clasts that are visible at HiRISE-scale resolution, which makes it dif- Fig. 1. HiRISE false-colour infrared images (IRB) of central uplifts with dykes at 25 cm/pixel. A) Thin dark-toned fractures and a thick dark-toned breccia dyke with a xenolithic clast indicated by the arrow (PSP_007767_1970). B) Cross-cutting blue- and light- toned breccia dykes in the uplift of Negril Crater (ESP_025700_2005). C) A 50 – 100 m thick green-toned dyke in the central uplift of Jori Crater with a variety of morphologies (ESP_028535_1515) compared to the geometries of offset dykes at the Sudbury impact structure. .
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